The ability to detect processor or subsystem faults within a multiple processor data processing system is essential to prevent the corruption of data or the malfunction of other components within the data processing system. Several fault diagnostic methods for use in multiple processor data processing systems are known in the art. Fault detection systems are described, for example, in "Decentralized Processing Method and System", Mori et al., U.S. Pat. No. 4,627,055, issued Dec. 2, 1986; "Remote Maintenance System for a Digital Communication Network", Kawano et al., U.S. Pat. No. 4,870,638, issued Sep. 26, 1989; and "Fault Diagnostic Distributed Processing Method and System", Mori et. al., U.S. Pat. No. 4,872,165, issued Oct. 3, 1989.
U.S. Pat. Nos. 4,627,055 and 4,872,165 each disclose distributed processing systems including a plurality of subsystems connected into a loop transmission system. Each subsystem has diagnostic means for identifying faults in other subsystems and on the network by placing messages onto the loop and monitoring the return to the messages. One disadvantage of the described system is that diagnostic operations interfere with other network operations.
U.S. Pat. No. 4,870,638 shows a digital communication network comprising a central toll center (TC) and several local switching offices (LOs), each LO being connected directly to the toll center by digital transmission lines. A central maintenance and operation center (CMOC) located at the toll center diagnoses alarm signals received from the LOs and sends maintenance signals for repairing identified faults to the LOs. Communication between the CMOC and the LOs can be accommodated by a system of dedicated data lines, by a sharing of the digital transmission lines which connect the LOs with the TC, or through the use of both the digital transmission lines and dedicated data lines. Diagnostic systems utilizing the digital transmission lines to communicate with the LOs are disadvantaged in that non-diagnostic communicates between the TC and LOs may be impaired during diagnostic operations.The system shown which utilizes a dedicated network of data lines for diagnostic communications does not provide redundant diagnostic communication paths as provided in the system employing both digital transmission lines and dedicated data lines or in the loop systems described in U.S. Pat. Nos. 4,627,055 and 4,872,165.
A diagnostic system in which diagnostic access to individual processors within a multiple processor system is provided though a redundant network of communication links independent of normal communication channels has been proposed to obviate the above-described problems.